In various communications systems, carrier sensing alone or in combination with a randomized back-off transmission time technique is used to determine when a device will transmit on a physical communications channel.
Consider, for example, 802.11 based systems. In a typical 802.11 based system, a broadcast packet is transmitted on a channel based on a DCF (distributed coordination function) mechanism. The system may include multiple nodes. Each node maintains a back-off counter used in determining when the node may transmit data within an overall time interval in which devices may transmit data signals to one another. The back-off counter is initialized to zero.
Each node that wishes to broadcast senses the channel and transmits if the channel is sensed to be idle for more than a duration known as DIFS and its back-off counter has expired. After each transmission, the nodes pick a new back-off counter. If the back-off counter value expires before the next packet arrives, the device can transmit after sensing the channel to be idle for a DIFS duration. (If the previous reception is unsuccessful, the device needs to wait for EIFS).
Typical drawbacks with this scheme are that all the waiting nodes with zero back-off counter choose to transmit at the same time when the channel becomes idle. Even if all waiting nodes have non-zero random back-off, the probability that 2 transmitters transmit at the same time is high (this can occur when two transmitters choose the same back-off), when the node density is high. Note that the spatial configuration of concurrent transmitters are not controlled by any protocol, i.e., the locations of the colliding transmitters can be arbitrary. This further leads to poor performance in receiving the broadcasting messages, especially in a dense deployment.
In view of the above discussion it should be appreciated that there is a need for improved methods of controlling when and/or how devices contend for transmission resources. In particular, it would be desirable if methods and apparatus which facilitate synchronized transmissions could be developed which could be used for one or more applications in a system which supports the use of carrier sensing.